I’m making a 54×18 frame and panel cabinet. I’m going to put a bookcase on top of it. The book case is 38×12 and will line up with the rear and right sides of the cabinet. Most of the weight will be on the 4 posts of the bookcase. I’m concerned about all that weight. I’ve considered buying a 1 inch thick plate of steel or gluing up a butcher block top with the end grain all pointing up.

Yeah, hard to envision. The butcher block top could take more weight if the grain was oriented lengthwise to the cabinet. A 1” thick piece of steel seems extreme, more like the deck of an aircraft carrier than a cabinet.

Last year I built a “bookcase” for my wife that is larger than that (72×30 base, 72×18 top). But it’s really four cabinets joined, so the middle of that length has great support (two base cabinet sides screwed together). The base cabinet top is hardwood-edged maple ply.

I would be concerned, in your case, about support for the upper cabinet in the middle of the length. But I’d just add some vertical support structure to the base cabinet in the middle of the legth. I would not add steel.

However, as Clint said, gotta have a picture to offer a pertinent suggestion.

No top rail in back ,huh? I’d consider adding one (supported on blocks glued and screwed to the back legs). Then, if the top is at least 5/4 (I might use 6/4), I believe it would be able to support an upper set of shelves (three, maybe four – so long as they are not too densley packed).

You could fit a vertical post, inside, back, beneath the new top rail, but I’m thinkin’ that’s unnecessary.

You are correct to be concerned about sagging in the middle, especially with a book case added to the top, and even more so if you fill it with books.

I have on occasion used angle iron myself and in your case behind/inside the top front rail. You could also use a piece of hardwood like a 2×6 tucked into the same spot. Otherwise it might be wise to put a centre stile in the middle behind the doors, and extend another leg under it

It’s hard to see what the back is made of if it’s a 1/2” thick or more and attached properly you may be ok but considering you have a 50” span or more I would install at least a 3/4” by 4” horizontal member to the top edge of the back . The weight of the upper book case will rest mostly on you 4 legs but you do need to be concerned about the span. you need to be concerned about the base cabinets constitution more than the top, if the top is supported properly a 3/4” thick top will be more than enough. All of my concerns are based on the book shelf being filled with books rather than just lighter displayed items.